Change grading algo to work with Submission entity

[ArthurCRodrigues]

Previously, we treated submission files as dict where key=filename and value=file content.

Now, we'll be working with the Submission entity to make things more organized, so the grader has to be adjusted for that.

Feel free to add any method or extra attributes to the new entity as you want.

Here is the Submission entity:

@dataclass
class SubmissionFile:
    filename: str
    content: str

@dataclass
class Submission:
    username: str
    user_id: int
    assignment_id: int
    submission_files: List[SubmissionFile]